Magnetic transducer with wear resistant pole tips

ABSTRACT

A composite magnetic transducer head for operation at frequencies in the megahertz range having pole tips made of specially fabricated laminations formed from ingots of fusion cast or sintered iron-silicon-aluminum alloy.

m u t it r it States 1 t1 Inventor Robert A. Schneider Del Mar, Calif.App1.No. 23,157 Filed Mar. 27, 1970 Patented Oct. 19, 1971 Assignee SpinPhysics, 1111c.

San Diego, Calif.

MAGNETIC TRANSDUCER WITH WEAR RESISTANT POLE TIPS 5 Claims, 8 DrawingRigs.

US. Cl 179/ 100.2 C, 29/603 lint. Cl G1 1b 5/412, G1 lb 5/14,G1 lb 5/22lField of Search 179/ 1 00.2

C; IMO/174.1 F; 346/74 MC; 29/603 [56] References Cited UNITED STATESPATENTS 2,861,135 11/1958 Rettinger 179/100.2C 2,992,474 7/1961 Adams eta1. 179/1002 C 3,499,214 3/1970 Schneider 29/603 OTHER REFERENCESMagnetic Materials in Electrical Ind. P.R. Bardell- 1960, MacDonald &Co. Ltd., London. pg. 96 LC. 0T1 453B3 Primary Examiner-Terrell W. FearsAssistant Examiner-.1 ay P. Lucas Attorney-Hill, Sherman, Meroni, Gross& Simpson ABSTRACT: A composite magnetic transducer head for operationat frequencies in the megahertz range having pole tips made of speciallyfabricated laminations formed from ingots of fusion cast or sinterediron-silicon-aluminum alloy.

MAGNETI'IC NSDIJCER wrm we assist" more it:

BACKGROUND OF THE INVENTION The fabrication of composite instrumentationheads is dealt with in my prior US. Pat. Nos. 3,417,209 and No.3,417,386 both dated Dec. 17, 1968. These patents disclose solid poletips for composite heads which are formed of a cast alloy material knownas sendust. There are numerous statements in the prior art to theefi'ect that sendust cannot be machined by conventional methods. See forexample US. Pat. No. 2,988,806, column 1, lines 19-31 and column 5,lines 1-4, and US. Pat. No. 2,992,474, column 2, lines 5449. So far aspresently known, heads utilizing an iron-silicon-aluminum material inthe as-cast condition have always utilized a solid pole tipconfiguration such as shown in my prior patents. The concept has been toutilize pole tips of small dimensions so as to minimize the detrimentaleffects thereof at high frequencies.

be fabricated with a thickness of 10 mils (l mil-0.001 inch), there isno suggestion in the known prior art that such plates of sendust with athickness of not more than about 0.2 mm. (approximately 8 mils) would besufficiently smooth and flat to enable the production of laminated poletips with uniform clearance therebetween of less than 300 microinches lmicroinch=0.000 l inch), so as to provide a laminated pole tipconfiguration.

Indeed in developing the head of the present invention it was found thatlaminations of the desired thinness (about mils) tended to curl andexhibit a potato chip effect. Further after annealing, the surfaceroughness increased by to 100 times, so that the laminations when placedat the required nominal separation (about. 200 microinches) hadelectrical contact with each other. Elaborate and extensive experimentwas required to develop techniques for overcoming these effects and todevelop a composite head with iron-silicon-aluminum pole tips exhibitingmarkedly improved highfrequency response in comparison to priorcomposite heads with solid sendust pole tips.

SUMMARY OF THE INVENTION This invention relates to an improved compositemagnetic transducer head capable of handling frequencies in themegahertz range and to specially fabricated laminations and pole tipconfigurations for such a head, and to a method of manufacturing thesame.

In the head construction of the present invention, the pole tips eventhough made of an extremely hard material, specifically a castiron-silicon-aluminum alloy having a hardness of at least Rockwell(3-40, have been successfully formed into a laminated configurationwhich is particularly advantageous for composite heads operating in themegahertz frequency range.

It is therefore an important object of the present invention to providean improved composite magnetic transducer head construction providingsubstantially better overall operating characteristics than prior artcomposite heads.

It is a further object of the present invention to provide a compositemagnetic transducer head providing a laminated pole tip configuration ofgreatly improved gap stability and uniformity during a substantiallyincreased useful life.

Further objects and features of the present invention relate to animproved lamination and an improved pole tip configuration, and tomethods and techniques for economically fabricating the same, withcommercially acceptable yields, and for reliably and convenientlyproducing a composite magnetic transducer head meeting desired frequencycharacteristics.

The laminated tip head of the present invention provides a frequencyresponse characteristic which is much less affected by head wear; alsothe head inductance is readily controlled to match any desired systemrequirements. With the solid sen- While certain prior art documentssuggest that sendust can dust tip head the length of the front gapbetween the ferrite core parts is very critical and the frequencyresponse characteristics of the head change drastically with wear(necessitating annoyingly frequent circuit adjustments in use of thehead). With the present laminated tip head the ferrite front gap can bewidened substantially {c.g. by 5 mils) without the head high-frequencyresponse falling below existing specifications.

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof, taken in conjunction with the accompanying drawings, althoughvariations and modifications may be efiected without departing from thespirit and scope of the novel concepts of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 11 is a partial somewhatdiagrammatic top plan view of a head assembly in accordance with thepresent invention;

FIG. 2 is a partial side elevational view of the head of FIG.

FIG. 3 is a somewhat diagrammatic vertical sectional view takengenerally along the plane represented by the line Ill-III in FIG. 1 andlooking in the direction of the arrows, but illustrating the headassembly prior to completion of certain of the fabricating steps;

FIG. I is a somewhat diagrammatic longitudinal sectional view takengenerally along the line lV-IV of FIG. 3 and illustrating in detail onlythe left half of the partially fabricated head assembly, a portion ofthe right half of the head assembly being indicated in dot-dash outline;

FIG. 5 is an enlarged somewhat diagrammatic longitudinal sectional viewillustrating the pole region of the completed magnetic head assembly ofFIG. 1 and! 2 and showing the path of a tape record medium thereacross;

FIG. 6 is a flow diagram setting forth certain principal steps in themethod of forming pole tips for the head of FIGS. 1-5 from acommercially available ingot of fusion cast iron-siliconaluminum alloy;

FIG. 7 is a somewhat diagrammatic top plan view illustrating thearrangement for carrying out the lapping operation of step number 3 ofFIG. 6; and

FIG. 8 is a somewhat diagrammatic vertical sectional view takengenerally along the plane represented by the line Vlll-Vlll in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIGS. 1 and 2 isillustrated a magnetic transducer head formed of a pair of headsubassemblies ll and 112. FIG. I shows a plan view of the tapeconfronting surface of the head which is formed by tip plates 13 and M,the top edges of shields -19 and the pole tips of transducer head units211-24. The coupling gaps for the respective head units are indicated at25-28 and are arranged to coincide with the interface between the headsubassemblies II and 12.

For the sake of diagrammatic illustration, a magnetic tape record mediumis indicated at 30 in FIGS. 2 and 5, having a flexible nonmagneticbacking 31 and a magnetizable layer 32 which is in sliding contact withthe surface of the head assembly 10 as represented in FIG. 5. As shownin FIG. 5, the magnetic tape record medium 30 moves in the direction ofarrow 33 across the pole tips 34 and 35 of each of the head units.Ferrite core parts are partially indicated at 36 and 37 in FIG. 5, and acomplete core part 36 is shown in FIG. 4. The core parts 36 and 37 haveconverging faces 36a, 37a, FIG. 5, which define a front nonmagnetic gap38, and have parallel confronting faces 36b, 37b, FIG. 4, defining aback nonmagnetic gap.

FIGS. 3 and 4 show the head subassembly 11 prior to completion ofcertain of the fabricating steps. For ease of reference, however,corresponding reference numerals have been applied to the parts in FIGS.3 and 45 which correspond to those in FIG. 1 and 2, even though certainof the parts such as the tip plate 13 as shown in FIGS. 3 and 4 willreceive further machining operations prior to completion of the head asshown in FIGS. 1 and 2. Referring to FIGS. 3 and 4, it will be observedthat the core sections such as 36 receive respective transducer windingssuch as those shown at 41-44 in FIG. 3. The configuration of each of thehead units 21-24 may be identical.

A mounting means for the ferrite core parts and shields comprisesmounting brackets 46 (FIGS. 3 and 4) and 47 (FIG. 2), having interiorwalls such as 46a, 47a, FIG. 5, defining mounting slots for receivingthe ferrite core parts 36 and 37. In forming the subassembly ll of FIGS.3 and 4, the core parts 36 are secured in the recesses of the associatedbracket 46 by a suitable cement. The edge faces 360 of ferrite coreparts 36 are lapped flat with lands such as indicated at 49, FIG. 3, ateach side of bracket 46.

A coating of cement is then applied to each ferrite core edge face 36c,and to each bracket land area as 58, FIG. 2, 49, FIG. 3, and 53, FIG. 4.The tip plate 13 is applied with sufficient pressure such that thecement bonds to the lower edges of the pole tip laminations and forms anelectrical insulating layer 50 of controlled thickness, for exampleabout 20 to 40 microinches. The excess cement is squeezed out frombetween the confronting surfaces of the ferrite core parts and therespective pole tips and into the intervening spaces such as indicatedat 51. The bracket 46 is shown with the land areas 49, FIG. 3, and 53,FIG. 4, spaced from the tip plate 13 by cement layers 52 and 54 whichare substantially equal in thickness to the layer 50. The insulating andbonding layers 50, 52 and 54 appearing in FIGS. 3, 4 and 5, andcorresponding layers such as 55, FIG. 5, and 56, FIG. 2, of the otherhead subassembly 12 are shown with exaggerated thickness in the drawingsfor the sake of clarity. The pressure and/or temperature applied.

to the bonding layers such as 50, 52, 54, 55 and 56 during the formationthereof may be selected to control their thickness dimension.

It has been found that the bonding layers 50 and 55, being electricallynonconductive in comparison to the conductivity of the ferrite edgefaces 36c and 37c, provide electrical isolation of the laminations ofeach pole tip assembly, and result in improved high-frequency responsewhere the core parts 36 and 37 are of a manganese zinc ferrite having aresistivity of the order of 1000 ohm-centimeters or less.

The interior spaces such as 51, FIG. 3, and 59, FIG. 4, between theundersurface 13a of the tip plate 13 and an upper surface 46b of bracket46 may be filled with a suitable epoxy supplied at apertures such as 60,FIG. 4, as described in my US. Pat. No. 3,4 I 7,209. The headsubassembly 12 is provided with similar apertures such as 61, FIG. 2, sothat spaces such as 62, FIG. 2, are likewise filled with epoxy whichsecures tip plate 14 to bracket 47. The further processing steps of thehead subassemblies may conform with those disclosed in my prior patent.

The shields 15-19 are seated in recesses such as indicated at 64, FIG.3, and may be bottomed against shield stops such as 65, FIGS. 3 and 4,for convenient positioning thereof during mating of the subassemblies 11and 12. After the parts 1 1 and 12 are assembled, the empty spaceswithin the assembly are filled with a suitable potting compound, and thetape contacting surface is fonned as indicated in FIGS. 1, 2 and 5.

By way of example, the gaps 25-28 between pole tips such as 34 and 35,FIG. 5, of head units 21-24, may each have a length dimension of about30 microinches for use as a playback head, and may have a lengthdimension of about 80 microinches for use as a record head. Theconfronting polar faces of pole tips 34 and 35 may have a depth orvertical dimension as viewed in FIG. 5 of about 35: mils, (1 mil equals0.001 inch), and the pole tips may each have a length of about 0.32 inchfor a playback head and about 0.15 inch for a record head. The front gap38 between the ferrite core parts may have a length dimension of theorder of 3 mils. (In each case the length dimension is generallyparallel to the direction of movement of the tape record medium.)

EXAMPLES OF LAMINATION FABRICATION TECHNIQUES Referring to FIG. 6, thebasic steps in an exemplary method for fabricating laminations from thecommercially available fusion cast material known as sendust areidentified by numerals l-9. The following numbered sections will referto these respective basic steps.

l. Sendust is commercially available in ingots, for example of lA-inchdiameter and 6 inches long. A typical example of such commercial alloyhas a composition of 84.5 percent iron, 6 percent aluminum and 9.5percent silicon and a mechanical hardness of Rockwell C42. The statedcharacteristics are as follows: density 7.1 grams per cubic centimeter,and electrical resistivity micro-ohm centimeters.

The ingots are cut into bars 1 inch wide and 0.350 inches thick, and aslong as possible, for example by an abrasive grinding technique using asilicon carbide wheel.

2. It is found that such bar material can be sliced into wafers having athickness of 8 mils (0.008 inch) by means of a Norton Wafering Machine,which is known per se in the semiconductor art. For the presentapplication, an aluminum oxide slurry is flowed over the blades of themachine which operate to progressively wear away material during a verylarge number of successive passes adjacent an edge of the bar. Thus,each bar is progressively sliced into a large number of wafers whichwhen viewed in side elevation are of rectangular configuration with alength dimension of 1.00 inch and a height dimension of 0.350 inch. Whenviewed in a particular orientation, each wafer may be provided with abeveled upper left-hand edge which serves to provide a reference markduring subsequent processing. The bars formed in step number 1 may beprovided with a suitable 45 chamfer along one of the long edges thereofso that the wafers are then formed with the beveled edge. (As anexample, a bar of pole tip material as formed in step number 1 may havea long dimension of 4 inches).

3. The wafers as formed in step number 2 are then subjected to a singlesurface lapping operation which serves to reduce the thickness of thewafers to the desired ultimate lamination thickness, for example between5 and 6 mils. It is found that such lapping can be effected utilizingaluminum oxide as the lapping compound and utilizing a Meehenite castiron plate or a hard steel plate. Further details of the lappingprocedure are given hereinafter having reference to FIGS. 7 and 8 of thedrawings.

4. The pole tip wafers are then placed between aluminum oxide plates(0.1 inch thick) with weights on top of the aluminum oxide plates totend to prevent curling or the "potato chip effect during an annealingoperation. As indicated in FIG. 6, the annealing operation may takeplace at l000 C. for 5 hours, with cooling thereafter at the rate of 50C. per hour until the temperature is reduced to 350 C. or 400 C., (whichis below the Curie temperature of 482 C.). The material can then beforced cooled. 1

It is found that the pole tip wafers after annealing developed verysubstantial surface roughness, which is believed to be due to crystalshift during the annealing process.

5. It is found possible to effectively remove this surface roughness bysubjecting the pole tip wafers to a further lapping operation to removesurface irregularities, and then subjecting the pole tip wafers to asecond annealing operation which may be identical to the first annealingoperation just described. It is found that the surface roughness doesnot again develop after the second annealing operation, and that thepole tip wafers now remain essentially flat and smooth with microinchoverall flatness and a surface finish of 4 microinches R.M.S.

With the resultant pole tip wafer arranged with the beveled edge at theupper left as before, the opposite surfaces of the wafer are inspectedover a zone between 20 and 60 mils below the upper edge to insure thatthis critical zone is free of pits greater than 0.0002-inch diameter(except adjacent the opposite vertical edges of the pole tip wafer).

Without the second annealing operation, it was found that thecommercially available material developed such roughness that a stack oflaminations with spacing of 200 microinches developed short circuitsbecause of the surface roughness.

6. The flat and smooth pole tip wafers as formed in step number 5 arethen stacked to form a wafer lamination assembly having a lengthdimension of 1.00 inch, and a height dimension of 0.35 inch and having,for example nine laminations with interposed cement layers. by way ofexample, the pole tip wafers may be sprayed or coated with a semirigidepoxy system and stacked in a fixture with accurate spacing. A hydraulicpress with heated platens then compresses the stack to the proper widthdimension with the excess epoxy being squeezed out from between thelaminations and a strong mechanical bond being formed between thesuccessive laminations of the stack to form a unitary structure, thebonding layers having a thickness of about 0.0002 inch (200 microinches)and less than 0.0003 inch (300 microinches).

7. The stack as formed in step number 6 is then double surface lapped toprovide the desired stack width, for example 0.0503 inches plus or minus0.0001 inch where the pole tip wafers had a thickness of 0.0056 plus orminus 0.0002 inch.

The wafer lamination assembly as thus formed is edge lapped along thelong (1.00 inch) upper edge face to remove edge roughness.

8. Where recording pole tops are to be formed, the wafer laminationassembly with the orientation just described is cut horizontally and thecut edges lapped to provide two wafer lamination assemblies each havinga height dimension of 0.155 inch and a lapped edge face which isperpendicular to the side surfaces thereof.

9. The wafer lamination assemblies are then cut into pole tip sectionshaving a depth dimension of 0.014 inch plus or minus 0.002 inch, with anedge face having a width dimension of 0.0505 inch and depth dimension of0.014 inch being formed from the lapped surface of the wafer laminationassembly so as to provide a gap defining face of the pole tip. Each poletip section is inspected for a distance of 0.06 inch from the lapped gapdefining edge face to be certain that no voids or pits greater than0.0002inch diameter can be seen at the side surfaces of the pole tipsection.

FIGS. 7 and 3 illustrate a suitable lapping apparatus 70 for carryingout step Nos. 3 and d. The lapping plate '71 is shown as rotating in thedirection of arrow 71?; on a central vertical axis. Four retaining ringsId- 77 have external gear teeth (not shown) meshing with a central gear7h which is selectively engagcabie to rotate with the lapping plate Tilas indicated by arrow 79. When the gear 7% is engaged, the retainingrings are driven clockwise, while when the gear 7% is disengaged,frictional contact of the rings "I' l-7'7 with the surface of thelapping plate 7i drives the rings in the counterclockwise direction. Thereversing of the direction of rotation of the rings I' l-7'7 is effectedas necessary to maintain the surface of the lapping plate substantiallyflat.

The four retaining rings such as M are held in place on the lappingplate ll during a lapping operation by means of four air cylinders (notshown) fixed to the upper frame of the apparatus and having respectivepiston rods such as indicated at hi, FlG. ti, carrying universallymounted pressure plates such as m which fit within the respective ringssuch as M.

in the illustrated embodiment, the wafers such as 0 are carried inconforming apertures such as 85a of circular work holdem such as 05.Circular disks 67-90 are secured to the op surfaces of the respectivework holders. The weight of the disks plus the pressure exerted by thecylinders may be such as to produce a pressure on the surfaces of thewafers such as 04 and as which engage the lapping plate of 6 pounds persquare inch.

As indicated in H6. 0, the thickness of the holder plate 85 may be 4mils, in comparison to an original thickness dimension of the wafers of8 mils. There may be 22 apertures such as @511 disposed in a radialpattern as indicated diagrammatically at M in FIG. '7.

The wafers can be loaded into each work holder while the work holdersare inverted. if the wafers are wetted with the slurry used duringlapping prior to insertion thereof into the apertures of the workholder, surface tension will retain the waters in the apertures as theholders are placed on the lapping plate 711.

The slurry may comprise aluminum oxide particles with a size of 25microns. One pound of the particles is mixed with 1 gallon of a suitablelapping oil such as Speedfam lapping oil supplied by Speedfam Corp,North Third Avenue, Des Plaines, Ill. The slurry is supplied in advanceof each retaining ring at a rate of a couple of drips per second.

The wafers are lapped for about 2 minutes on one side and then inverted.This cycle is repeated about three time so that the total processingtime for each group of lid wafers is about 12 minutes.

The lapping plate Ill may be rotated at about 50 revolutions per minute.

in the double surface lapping operation of step No. 7, lapping platesengage the stacks on each side of the work holders. A slurry may beformed using aluminum oxide particles of 3 microns, and a mixture of 1pound of particles per gallon of the same lapping oil as before. About10 minutes is required for this lapping operation.

The broad teachings of the present invention are also ap plicable toingots of sintered silicon-aluminum-iron alloy having a thicknessinitially of at least about 20 mils and having a mechanical hardness ofat-least Rockwell C- lO. For example, one sintered silicon-aluminum-ironalloy of approximately sendust composition has a mechanical hardness ofRockwell C-48, 21 resistivity of approximately 2.00 micro-ohmcentimeters, a coercive force from saturation of 0. 10 oersted, a suturetion induction of ,9000 gausses, and initial and maximum permeabilitiesof 6500 and 25000, respectively. The concept in the prior art has beento sinter compacted masses of such alloys having essentially the desiredultimate thickness. Following the teachings of the present invention,blocks or bars of the sintered material can be formed and then wateredand lapped as taught herein to provide laminations with a thickness ofabout 5 to 6 mils, and having much greater density and better magneticproperties than laminations heretofore available from this material.

In the case of one particular sintered alloy, an ingot 2 inches indiameter and 2 inches long was formed, having a Vickers hardness of theorder of 700 (with 300 gram loading) in comparison to a Vickers hardnessfor a fusion casting of the conventional sendust material of about .500,(sendust having a Rockwell hardness of about CZ-42). This sintered alloyhad a resistivity of about to l 10 micro-ohm centimeters and was waferedand lapped as taught herein to provide a lamination thickness of about5%: mils. The wafers were annealed in a vacuum furnace by raising thetemperature of the heating chamber from room temperature to 800 C infrom one-half hour to l hour, and then shutting off the furnace andallowing the heating chamber to cool back to room temperature at anunforced rate with the chamber remaining closed).

in this case the annealed wafers were briefly dipped (for about 5seconds) into a viscous hydrofluoric acid solution to etch the surfacesthereof. This rapid dip acid treatment step removes any silicon whichmay have diffused to the surface of the wafers without weakening thewafers by excessive etching at the grain boundaries. The resultingsurface is found to be chemically clean as verified by a water breaktest.

With this latter sintered alloy the wafering procedure was the same asfor the fusion cast material, but it was found that the wafers did notdevelop the severe degree of roughness after annealing experienced withthe cast material, and consequently the reannealing step could beomitted. it is theorized that a sintering operation introduces lessstress in the material than fusion casting, so that the annealinginduces markediyless crystal shift.

The term ingot is used herein to refer to the product of a conventionalfusion casting process and to the analogous product formed by compactingand sintering of powdered alloy material. The term denotes a producthaving the typical density of the fusion cast material or a productformed by sin tering but having comparable density. The term "ingot" asused herein requires an iron-silicon-aluminum alloy mass having adensity comparable to that of fusion cast sendust such that the Rockwellhardness thereof is at least C40. The term ingot" requires a density ofa sintered material of about sendust composition of at least 6.5 gramsper cubic centimeter. Sintered materials of this density may have adirect current permeability (measured with A H equals 0.02 oersted on aring sample with a thickness of 0.350 inch) of at least 6000, thispermeability being defined as initial permeability herein.

I claim as my invention:

1. A composite magnetic transducer head comprising a ferrite magneticcore having magnetic pole tips defining a coupling gap for coupling ofthe core with a record medium, characterized in that at least one of thepole tips comprises laminations formed from an ingot of aniron-silicon-aluminum alloy, said laminations having a mechanicalhardness of at least Rockwell C-40 and a thickness of about to 6 mils,said laminations having substantially smooth side surfaces, and abonding layer between the successive laminations and mechanicallybonding the laminations as a unitary structure, the successive bondinglayers having a substantially uniform thickness dimension of about 200microinches, said laminations having a density of about 7 grams percubic centimeter and an initial penneability of at least about 6500.

2. A magnetic head comprising a magnetic core having magnetic pole tipsdefining a coupling gap for coupling of the core with a record medium,characterized in that the pole tips comprise laminations formed fromfusion cast iron-silicon-aluminum alloy, said laminations having ahardness of at least Rockwell C-40 and a thickness of less than 8 mils,said laminations having lapped side surfaces which are substantiallyflat and smooth, and a bonding layer between successive laminationsmechanically bonding the laminations as a unitary structure and having athickness of less than 300 microinches, said laminations having aninitial permeability of at least about 6500.

3. A magnetic head according to claim 2 with said laminations of saidfusion cast alloy having been annealed, lapped smooth on the sidesurfaces thereof and then reannealed.

4. A composite multitrack magnetic transducer head assembly comprising asupport bracket having a series of ferrite magnetic cores therein, thecores having respective pairs of edge faces all lying substantially in acommon plane,

a pole tip assembly having secured thereto a series of pairs of poletips with undersurfaces lying in closely spaced confronting relation tothe respective edge faces to define respective head units forcooperation with respective channels of a magnetizable layer of amagnetic record medium.

said pole tips each comprising a series of laminations formed from aningot of a sintered iron-silicon-aluminum alloy and having a hardness ofat least about Rockwell C-40 and having a thickness of about 5 to 6mils, said laminations having a density of at least 6.5 grams per cubiccentimeter and an initial permeability of at least 6000.

5. A multitrack magnetic transducer head assembly according to claim 4with said laminations having been etched with a viscous acid solutionsubsequent to annealing, and the laminations of each pole tip assemblyhaving successive bonding layers therebetween mechanically bonding thelaminations as a unitary structure and providing a substantially uniformspacing between the successive laminations of less than 300 microinches.

1. A composite magnetic transducer head comprising a ferrite magneticcore having magnetic pole tips defining a coupling gap for coupling ofthe core with a record medium, characterized in that at least one of thepole tips comprises laminations formed from an ingot of aniron-silicon-aluminum alloy, said laminations having a mechanicalhardness of at least Rockwell C-40 and a thickness of about 5 to 6 mils,said laminations having substantially smooth side surfaces, and abonding layer between the successive laminations and mechanicallybonding the laminations as a unitary structure, the successive bondinglayers having a substantially uniform thickness dimension of about 200microinches, said laminations having a density of about 7 grams percubic centimeter and an initial permeability of at least about
 6500. 2.A magnetic head comprising a magnetic core having magnetic pole tipsdefining a coupling gap for coupling of the core with a record medium,characterized in that the pole tips comprise laminations formed fromfusion cast iron-silicon-aluminum alloy, said laminations having ahardness of at least Rockwell C-40 and a thickness of less than 8 mils,said laminations having lapped side surfaces which are substantiallyflat and smooth, and a bonding layer between successive laminationsmechanically bonding the laminations as a unitary structure and having athickness of less than 300 microinches, said laminations having aninitial permeability of at least about
 6500. 3. A magnetic headaccording to claim 2 with said laminations of said fusion cast alloyhaving been annealed, lapped smooth on the side surfaces thereof andthen reannealed.
 4. A composite multitrack magnetic transducer headassembly comprising a support bracket having a series of ferritemagnetic cores therein, the cores having respective pairs of edge facesall lying substantially in a common plane, a pole tip assembly havingsecured thereto a series of pairs of pole tips with undersurfaces lyingin closely spaced confronting relation to the respective edge faces todefine respective head units for cooperation with respective channels ofa magnetizable layer of a magnetic record medium. said pole tips eachcomprising a series of laminations formed from an ingot of a sinterediron-silicon-aluminum alloy and having a hardness of at least aboutRockwell C-40 and having a thickness of about 5 to 6 mils, saidlaminations having a density of at least 6.5 grams per cubic centimeterand an initial permeability of at least
 6000. 5. A multitrack magnetictransducer head assembly according to claim 4 with said laminationshaving been etched with a viscous acid solution subsequent to annealing,and the laminations of each pole tip assembly having successive bondinglayers therebetween mechanically bonding the laminations as a unitarystructure and providing a substantially uniform spacing between thesuccessive laminations of less than 300 microinches.